US3526767A

US3526767A - Image amplification in ionography by avalanche method

Info

Publication number: US3526767A
Application number: US676276A
Authority: US
Inventors: Walter Roth; Alex E Jvirblis
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1967-10-18
Filing date: 1967-10-18
Publication date: 1970-09-01
Anticipated expiration: 1987-09-01
Also published as: GB1228948A; NL6814788A; ES359265A1; BE722354A; BR6801784D0; FR1587025A; SE354362B; DE1803667A1; DE1803667B2

Description

Sept. 1, 1970 w, RQTH L 3,526,767

IMAGE AMPLIFICATION IN IONOGRAPHY BY AVALANCHE METHOD Filed Oct. 18, 1967 2 Sheets-Sheet 1 I I H F/G/ ll l0 /a /a 245;: qusgggme r/az l 50 J2\ as 7///// QUENCHING 7/ ////////////////////////////////A GAS 74L: INVENTORS WALTER ROTH rw-W// //A EALEX MP 68 a I a A T TOR/VEKS Sept. 1, 1970 w. ROTH ETAL 3,526,767

IMAGE AMPLIFICATION IN IONOGRAPHY BY'AVALANCHE METHOD Filed Oct. 18, 1967 2 Sheets-Sheet 2 v I FIG. 3

//l \gg I V j 90 QUENCHING GAS QUE NCHING l/Z GAS INVENTORS l/O WALTER ROTH ALEX E JVIRB IS k\\\\\\\\\\\\\\\\\ BY V/////////////////////////////////////////A A TORNEYS United States Patent 3,526,767 IMAGE AMPLIFICATION IN ION OGRAPHY BY AVALANCHE METHOD Walter Roth, Rochester, N.Y., and Alex E. Jvirblis, Santa Clara, Calif., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 18, 1967, Ser. No. 676,276 Int. Cl. G03!) 41/16 U.S. Cl. 25065 37 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates in general to X-ray recording and in particular to an improvement in X-ray recording by ionography.

The art of X-ray recording by Xerography, generally I known as xeroradiography, relates to the recording of X- ray patterns and information by means of materials and devices whose electrical conductivity is altered by the action of sensitizing radiation, such as X-rays and the like. In xeroradiography the plate or element exposed to X-ray or gamma ray or other sensitizing radiation usually comprises a conductive metallic backing sheet having a photoconductive insulating layer or coating, for example of vitreous or amphorous selenium on one surface thereof. It is conventional to cover or protect the coating from ambient light by a slide plate placed from the surface of the coating and usually called a dark slide. The plate or element is sensitized by applying an electrostatic charge to the coating and thereafter the sensitized plate or element is exposed to sensitizing radiation with the object to be radiographed appropriately interposed between the radiation source and the plate. Under influence of the radiation from the source which readily passes through the dark slide, the coating becomes electrically conductive permitting the electrostatic charge thereon to be selectively dissipated in those portions reached by the sensitizing radiation, less dissipation occurring in those portions of the coating shaded by the object being radiographed in proportion to its absorbed radiation. In this manner an electrostatic latent image of the radiographed object is formed on the coating. This image may then be developed or made visible with an electroscopic marking material which clings to the electrically charged portions of the latent image. Such a process is disclosed, for example, in Schaifert et al. US. Pat. No. 2,666,144.

In another form of radiography, the X-rays or other imaging radiation are utilized to differentially ionize air or other gas existing between a cathode supporting the object to be radiographed and the layer upon which the electrostatic latent image is to be formed.

In Criscuolo et al. US. No. 2,900,515 there is disclosed a gas-ionization radiographic process wherein a wire mesh is placed on the opposite side of the object to be radiographed from the X-ray source but between the object and a uniformly charged photoinsensitive insulating material. The Xrays not absorbed by the object cause differential ionization of the gas between the wire mesh and the object. This differential ionization causes the surface "ice charge to leak off the photoinsensitive insulating material in proportion to the amount of radiation received thereby forming a latent electrostatic image which can be rendered visible. No external potential is applied to the wire mesh and it is clear that charge is caused to leak off the photoinsensitive insulating material towards the wire mesh and not through the photoinsensitive insulating material layer.

Reiss, Zeit fur Angew. Physik, Vol. 19, pages 1-4, Feb. 19, 1965, discloses another imaging arrangement having a lead coated cathode connected through an external potential source to an aluminum anode having an insulating layer disposed thereon. This unit is placed in an airtype chamber which is then filled with a flowing quenching gas, for example, a one to one mixture of Freon and propane. Simultaneous with the exposure of the cathode to X-rays, a direct current potential is applied across the electrodes so that photoelectrons ejected from the lead layer are considerably intensified by an avalanching process in the quenching gas. The electrons are collected on the insulating layer on the anode in an image pattern corresponding to the intensity of the imaging radiation absorbed in the lead layer.

It is apparent that in the Reiss apparatus and technique, successive avalanche electrons or other image-defining negative charges which charge the insulating layer are impeded in their movement by the retarding fields created by the previously deposited negative charges. This limits the technique to a very small dynamic range which, in turn, limits image quality, sensitivity and resolution.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide a radiogr-aphic technique utilizing gas ionization which is not subject to the aforementioned deficiency.

It is an object of the present invention to provide a radiographic technique utilizing gas ionization wherein image-forming, avalanching charges are not subjected to retarding fields created by previously deposited charges.

It is a further object of the present invention for providing novel means for achieving the preceding objects.

It is still a further object of the present invention to provide novel radiographic imaging means wherein a low Work function or photo-emissive material is utilized to substantially counteract the efiect of the retarding fields created by deposited image forming charges.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific exemplary embodiments.

SUMMARY OF THE INVENTION The above and still further objects of the present invention are achieved by counter-balancing the deposition of a first polarity, image-forming avalanching charge with the simultaneous formation of an opposite polarity charge sufiiciently adjacent said first charge so that the field established thereby is substantially neutralized. For example, the net opposite polarity charge can be formed by the dissipation or ejection of an electron from a nearby or adjacent layer. In one embodiment as a negative avalanching electron or ion neutralizes an existing positive charge residing on an insulator surface, an electron is dis sipated by photoconductivity from the interface existing between the insulator and an adjacent photoconductive insulator. In a further embodiment, for each negative charge that is deposited upon an insulating film overlying an anode, an electron is ejected from a layer of photoemissive material coated on the opposite surface of said anode. In this manner of counterbalancing charge deposition with the formation of a net charge of opposite polarity, within the field of influence of the first charge, the

field existing between the anode and the cathode remains substantially constant throughout image formation. Having eliminated the retarding fields created by previously deposited charges, successive avalanching charges are not adversely influenced thereby.

Throughout the course of the specification reference will be made to charges which will be depositing on the insulator or photoconductive insulator surface. This term is meant to include, though not intended to be limited thereto, electrons and negative ions formed in the quenching gas. It is know that both are present and, in accordance with known electrostatic principles, both will be drawn toward the positive pole. As previously indicated, these charges will neutralize opposite polarity charge or merely deposit on the insulator surface, either effect taking place in image-forming configuration.

BRIEF DESCRIPTION OF THE DRAWINGS The nature of the invention will more easily be understood when it is considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional side view of the ionographic imaging apparatus illustrating the process of the present invention;

FIG. 2 is a cross-sectional side view of an alternative ionographic imaging apparatus;

FIG. 3 is a cross-sectional side view of still a further ionographic imaging apparatus; and

FIG. 4 is a cross-sectional side view of an imaging apparatus wherein the object to be radiographed is supported by a low work function material coated anode.

It should be understood that in all of the figures the thickness of the layers, electrodes, etc., have been greatly exaggerated to show the details of construction. No inferences should be drawn as to the relative thickness of the layers or the spacings separating the various elemental parts of the imaging apparatus.

Referring to FIG. 1, there is seen an ionographic imaging apparatus generally designated having an object, for example a step wedge 12, situated between X-ray source 14 and the remainder of the imaging apparatus 10. There is provided a cathode 16 having a coating 18 of photo-emissive material thereon. Exemplary photo emissive materials are lead and lead oxide. Coating 18 should be capable of ejecting electrons in response'to radiation absorbed in the layer. Parallel to cathode 16 and in spaced relationship thereto is anode 20 which is connected by lead 22 to one terminal of potential source 24, the other terminal of source 24 being connected by lead 26 to cathode 16. Disposed on the side of the anode 20 facing cathode 16 is a layer of photoconductive ins'ulating material 28 having an overlying insulating layer 30. Layer 28 of photoconductive insulating material must be responsive to (i.e. made conductive by) X-rays or other impinging radiation used in this imaging technique. Further, layer 28 must have suflicient mobility for negative charge residing at interface I so it will be transported to anode 20 without formation of a trapped bulk charge. An exemplary material is a thin selenium layer.

In operation, layer 30 is given a uniform positive charge over its entire surface, for example, by subjecting it to corona discharge as is known in the art. Simultaneously with the formation of the uniform positive charge, there will be an induced negative charge at the interface I between the insulating layer 30 and the photoconductive insulating layer 28. A quenching gas is admitted to the space between the electrodes so that amplification can be achieved during exposure. Though not shown, the apparatus can be enclosed in an air-tight container, or an equivalent design, whereby the quenching gas, if kept under a slight gas flow, can displace all air from the intervening space. Object 12 is now exposed, for a short interval, to radiation emanating from source 14. The radiation will be absorbed to a greater extent by the thicker areas of object 12 thereby permitting less radiation to be absorbed in the corresponding underlying areas of photoemissive layer 18. Radiation which is absorbed in layer '18, however, will cause said layer to eject electrons therefrom. These primary electrons will ionize the gas molecules and produce secondary electrons. At proper voltages avalanching takes place and secondary electron flow is greatly increased. These electrons, as well as negative ions, will be drawn across the gap between the electrons to insulating layer 30. If the gap between layers is small, resolution is only rninutely alfected and the negative charges will be deposited in imagewise configuration. Depositing negative charges neutralize the positive surface charge on insulator 30 while, simultaneously therewith, those X- rays which have passed through object 12, cathode 16, layer 18 and insulator layer 30 will discharge the photo-' conductive insulator layer 28. The intensity of the radiation reaching layer 28 will be inversely proportional to the thickness of various portions of object 12 and, of course, will be greatest where the radiation did not have to pass through the object prior to impinging upon layer 28. In those areas where the object is the thickest, more radiation will be absorbed and, therefore, there will be less radiation available to discharge layer 28. Correspondingly, there will be less primary electrons ejected from layer 18 to ionize intervening gas molecules and produce additional electrons. The converse, however, is true with respect to thinner portions of the object. More electrons will be ejected from corresponding portions of layer 18 and more radiation will be available to discharge layer 28. Accordingly, the simultaneously occurring effects operate to substantially reduce or eliminate the retarding fields created by successive depositions of image-forming. negative charges.

Referring to FIG. 2, ionographic imaging apparatus generally designated 50 has a cathode 52 having a photoemissive coating 54 disposed thereon. As in FIG. 1, object 12 is positioned between source 14 and cathode 52 and is, in fact, supported by the exposed surface of cathode 52. Spaced from the cathode and in parallel relationship thereto is anode 56 sandwiched between insulator layer 58 and photo-emissive layer 60, insulator layer 58 being on the cathode side of anode 56. Anode 56 is connected by lead 62 to one terminal of potential source 64, the other terminal being connected by lead 66 to cathode 52. On the opposite side of anode 56 from cathode 52 and in parallel spaced relationship thereto is a collector electrode 68. Collector electrode 68 is connected by lead 70 to the positive terminal of potential source 74, the negative terminal being connected to the positive terminal of potential source 64. As with the apparatus of FIG. 1, quenching gas is emitted to the space between the cathode and the anode. Optionally, quenching gas is admitted to the space between the anode and the collector electrode though it is not absolutely necessary. Since the apparatus of FIG. 2 does not include a photoconductive insulator, it can be operated in ordinary room light provided the photo-

emissive layers

54 and 60 are not sensitive thereto.

In operation, the apparatus of FIG. 2 does not depend on the neutralization of charge residing on the insulator surface and the simultaneous dissipation, by photoconductivity, of induced charge residing at the insulatorphotoconductive insulator interface. Rather, the apparatus of FIG. 2 neutralizes the retarding fields caused by successive deposition of image-forming avalanching charges by the simultaneous ejection of an electron from the insulator material. The net positive charge on the photo-emissive material substantially neutralizes the fields created by the negative, image-forming charges on the insulator material. In this manner, successive negative charges are not retarded whereby the apparatus achieves a greater dynamic range.

As with the apparatus of FIG. 1, the intensity of the radiation absorbed in the photo-emissive layer and the intensity of negative charges striking the insulating film is greatest where the photo-emissive material on the cathode is not beneath the object to be radiographed. The

intensity in areas covered by the object is inversely proportional to the thickness of the object since more radiation is absorbed by thicker areas. In response to radiation received thereby, electrons will be ejected from photoemissive layer 54 causing secondary electrons and other negative charges to be formed in the quenching gas. These negative charges will be deposited on insulating film 58 in image configuration. Simultaneous with the deposition of the negative charges on the insulating film, the radiation impinging upon the photo-emissive layer 60 on the opposite side of cathode 56 from insulator 58 will cause ejection of electrons therefrom. This will result in a net positive charge in image configuration on layer 60. Continued exposure to radiation in these areas will cause further ejection of electrons thereby increasing the net positive charge in more highly irradiated areas. The electrostatic field established by the net positive charge on layer 60 will substantially cancel or neutralize the effect of the net negative charge on the insulating film. In this manner, the retarding fields established by the negative imaging charges is diminished with the result that successive avalanching charges are not adversely influenced.

While the imaging apparatus of FIG. 1 produces a negatively charged image on the exposed surface of insulator layer 30, the apparatus of FIG. 2 produces a positively charged image on the exposed surface of photoemissive material 60 as well as the negatively charged image on the exposed surface of insulator layer 58. Image development can proceed in accordance with well known electrostatic and xerographic development techniques. Thereafter, the images can be transferred from the insulator or photo-emissive surfaces to a copy sheet. Alternatively, the insulator layer having the developed image thereon can be stripped from the anode and the image fixed directly thereto.

Referring to FIG. 3, there is seen an imaging apparatus 80 having an object, for example a step Wedge 12, situated between X-ray source 14 and the remainder of the imaging apparatus 80. There is provided a cathode 82 having a coating 84 of photo-emissive material thereon. Parallel to cathode '82 and in spaced relationship thereto is anode 86 which is connected by lead 88 to one terminal of potential source 90, the other terminal of source 90 being connected by lead 92 to cathode 82. Disposed on the side of anode 86 facing cathode 82 is a layer of insulating material 94 upon which the latent electrostatic image will be formed. Disposed on the opposite side-of anode 86 is a layer 96 of low Work function material. This latter material should have substantially higher electron mobility than hole mobility and retention time for the hole should be such as not to adversely affect subsequent image visualization. An exemplary material is barium zirconate.

In operation, electrons will be ejected from layer 84 and deposited on insulating material layer 94 in image configuration in proportion to the amount of radiation absorbed by object 12; that is, more electrons will be deposited in those layers corresponding to thinner portions of object 12. Those X-rays passing through to low work function material layer 96 will cause electrons to be ejected therefrom which will be collected by anode 86. This will leave behind positive holes, the effect of each will be to neutralize the retarding field created by a depositing negative charge.

Referring to FIG. 4, there is seen an imaging apparatus 100 having a step wedge 12 situated between X-ray source 14 and the remainder of the apparatus. There is provided an anode 102 having a coating 104 of low work function material disposed on the surface of anode 102 facing source 14. Disposed on the opposite surface of anode 102 is a layer of insulating material 106. In parallel spaced relationship to anode 102 is cathode 108 which is connected to the anode through potential source 112. Disposed on the surface of cathode 108 closest to anode 102 is a layer 110 of photo-emissive material.

In operation, electrons are ejected from layer 110 and attracted towards anode 102 and deposited on insulator layer 106 in imagewise configuration. Simultaneously, an electron is ejected from low work function material 104 and collected by anode 102 thereby creating a positive hole. The formation of the positive hole substantially neutralizes the retarding field created by the deposition of an image-forming negative charge.

In each of the preceding figures, quenching gas is admitted, as previously described, to achieve negative charge avalanching in the gap between the photo-emissive layer and the insulator layer.

DESCRIPTION OF SPECIFIC EMBODIMENTS In the following examples, the apparatus of FIG. 4 will be utilized wherein there is provided a steel cathode separated by a 20 mil gap from an aluminum anode. The photoemissive layer on the steel cathode is a dispersion of lead oxide (Pb O in an ethyl cellulose binder. Barium zirconate is the loW work function material dis posed on the anode surface and the image-defining charges are deposited on a 3 mil thick polyethylene terephthalate layer. An X-ray source is utilized in all examples to expose the apparatus through a step wedge.

In all examples, a control is run which differs from FIG. 4 in that the barium zirconate layer is omitted.

EXAMPLES I-II The control and the FIG. 4 apparatus are each exposed for one and one-half seconds to an X-ray source of kilovolts peak (kvp.) and milliamps with 800 volts potential applied across the electrodes. After image formation, on the polyethylene terephthalate film, the images are powder-cloud developed. Sensitivity of the image produced with the FIG. 4 apparatus is increased by approximately a factor of 3 over the control.

EXAMPLES III-IIV Examples I-II are repeated except exposure is limited to 0.20 second. Sensitiity of the image produced with the FIG. 4 apparatus is increased by approximately a factor of 3 over the control.

EXAMPLES V-VI Examples III are repeated except exposure is limited to 44 kvp. Sensitivity of the image produced with the FIG. 4 apparatus is increased by approximately a factor of 3 over the control.

EXAMPLES VII-VIII Examples V-VI are repeated except 1200 volts potential is applied across the electrodes. Sensitivity of the image produced with the FIG. 4 apparatus is increased by approximately a factor of 3 over the control.

While the invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the true spirit and scope of the invention. Such changes are considered to be within the scope of the present invention as defined by the claims appended hereto.

What is claimed is:

1. An ionographic imaging apparatus comprising means for depositing image-forming charges on an insulator surface in proportion to the intensity of imaging radiation absorbed by an object to be radiographed and means operatively associated with said first mentioned means for simultaneously substantially neutralizing the retarding fields created by said image-forming charges deposited on said insulator layer.

2. An ionographic imaging apparatus comprising a cathode and an anode in parallel spaced proximity thereto, said cathode being connected through an external potential source to said anode, said cathode having a layer of photo-emissi've material on its side adjacent said anode, said anode having an insulator layer adjacent its side closest to said cathode, said cathode and said anode being further apart than the combined thicknesses of said photoemissive layer and said insulator layer to thereby define a gap therebetween, and means operatively associated with said anode for substantially neutralizing the retarding fields created by image-forming charges deposited on said insulator layer.

3. The apparatus of claim 2 further including means for excluding air from and for introducing a volume of quenching gas into at least the gap between said cathode and said anode.

4. The apparatus of claim 3 wherein said means for introducing a volume of quenching gas between said cathode and said anode includes means for maintaining at least a gentle flow of said quenching gas in said gap.

5. The apparatus of claim 2 further including a source of imaging radiation operatively positioned to expose said cathode through an object to be radiographed.

6. The imaging apparatus of claim 2 wherein the neutralization means includes a photoconductive insulator layer sandwiched between said insulator layer and said anode, said photoconductive insulator layer being re sponsive to imaging radiation.

7. The imaging apparatus of claim 6 further including means to charge said insulator layer to a substantially uniform positive potential.

8. The imaging apparatus of claim 6 further including means to shield said photoconductive insulator layer from ambient, non-imaging electromagnetic radiation.

9. The imaging apparatus of claim 2 wherein the neutralization means includes a second photo-emissive layer disposed on the opposite surface of said anode from said insulator layer and a collector electrode in parallel spaced proximity thereto, said anode and said collector electrode being connected through a second external potential source of greater potential than said first potential source.

10. The imaging apparatus of claim 2 wherein the neutralization means comprises a layer of low work function material disposed on the opposite side of said anode from said insulator layer.

11. The apparatus of claim 10 further including a source of imaging radiation operatively positioned to expose an object to be radiographed supported by the exposed surface of said cathode.

12. The apparatus of claim 10 further including a source of imaging radiation operatively positioned to expose an object to be radiographed supported by the low work function material coated side of said anode.

13. The apparatus of claim 10 wherein the low work function material comprises Pb O 14. The apparatus of claim 10 wherein the low work function material comprises Pb O in a resinous insulating binder.

15. An imaging apparatus comprising a cathode and an anode in parallel spaced relationship thereto, said cathode being connected to said anode through an external potential source, said cathode having a layer of photoemissive material coated on the side of said cathode adjacent said anode, and said anode having a photoconductive insulator layer on the side of said anode adjacent said cathode sandwiched between said anode and an insulator layer overlying said photoconductive insulator layer, said cathode and said anode being further apart than the combined thicknesses of said photo-emissive layer, said insulator layer and said photoconductive insulator layer to thereby define a gap between said photo emissive layer and said insulator layer.

16. The imaging apparatus of claim wherein the external potential source is of sufiicient potential to achieve electron avalanching in a quenching gas admitted to said gap during imaging.

17. The method of substantially neutralizing the retarding fields created by deposited image-forming charges comprising providing an apparatus as disclosed in claim 15, charging said insulator layer to a substantially uniform positive potential, causing the deposition of said image-forming charges on said insulator surface so that positive charges thereon will be neutralized in imagewise configuration in proportion to the amount of imaging radi ation absorbed by the object being radiographed, and simultaneously therewith counter-balancing the deposition of said deposited charges with the dissipation of induced charges of same polarity as said deposited charges from the interface between said insulator layer and said photoconductive insulator layer.

18. The method of claim 17 wherein said induced charges are dissipated through said photoconductive insulator layer in response to imaging radiation received thereby.

19. The method of claim 17 further including visualizing the latent electrostatic image formed on said insulator layer.

20. The method of claim 19 further including fixing said visualized image to said insulator layer.

21. The method of claim 19 further including transferring said visualized image from said insulator layer to a copy sheet and fixing said image thereon.

22. A method comprising providing an apparatus as disclosed in claim 15, charging said insulator layer to a substantially uniform positive potential, causing a gentle flow of quenching gas to fill said gap, placing an object to be radiographed between a source of imaging radiation and the exposed surface of said cathode, exposing said object to imaging radiation while maintaining sufiicient potential across said cathode and said anode to cause negative charge avalanching in said quenching gas whereby image-forming negative charges will be deposited on said insulator layer in image configuration to thereby neutralize positive charges thereon in proportion to the amount of imaging radiation absorbed by said object, and simultaneously therewith dissipating negative charges at the interface between said photoconductive insulator layer and said insulator layer by photoconductivity in response to imaging radiation received by said photoconductive insulator layer.

.23. An imaging apparatus comprising a cathode and an anode in parallel spaced relationship thereto, said cathode being connected to said anode through a first external potential source, said cathode having a layer of photoemissive materials coated on the side thereof adjacent said anode, said anode having an insulator layer disposed on the surface thereof adjacent said cathode, said cathode and said anode being further apart than the combined thicknesses of said photo-emissive layer and said insulator layer to thereby define a gap therebetween, said anode having on the opposite surface thereof a sec ond layer of photo-emissive material, said anode being connected through a second external potential source to a collector electrode which is in parallel spaced relationship to said anode and said second photo-emissive material layer thereon, said second potential source being of greater potential than said first potential source so that said anode remains positive with respect to said cathode.

24. The imaging apparatus of claim 23 wherein the first external potential source is of sufficient potential to achieve electron avalanching in a quenching gas admitted to said gap between said first photo-emissive layer and said insulator layer during imaging.

25. An ionography imaging method comprising providing the imaging apparatus of claim 23, disposing an object to be radiographed between a source of imaging radiation and the exposed surface of said cathode, causing a quantity of quenching gas to be flowed through at least the gap between said cathode and said anode, exposing said object to imaging radiation while sufficient potential is maintained across said anode and said cathode to cause negative charge avalanching in the gap therebetween whereby negative image-forming charges are deposited on said insulator layer in imagewise configuration in proportion to the amount of radiation absorbed by said object, and simultaneously therewith causing said imaging radiation passing through to the photo-emissive layer disposed on the opposite surface of said anode from said insulator layer to eject electrons from said photoemissive layer whereby the net positive charge residing on said photo-emissive layer substantially neutralizes the retarding fields created by negative charges deposited on said insulator layer.

26. The method of claim 25 wherein the ejection of electrons from said photo-emissive layer on said anode causes the formation of a positive electrostatic latent image thereon, and further including visualizing said positive latent electrostatic image and transferring said visualized image from said photo-emissive layer to a copy sheet and fixing said image thereon.

27. An imaging method comprising causing the deposition of image-forming charges in imagewise configuration on an exposed surface of an insulator layer, charge deposition being pro ortional to the amount of imaging radiation absorbed by an object to be radiographed, and simultaneously therewith substantially neutralizing the retarding fields created by said deposited charges.

28. The method of claim 27 wherein the retarding fields created by said deposited charges are substantially neutralized by the formation of opposite polarity charges within the sphere of influence of said deposited charges.

29. The imaging method of claim 27 wherein the retarding fields created by said deposited charges are substantially neutralized by the dissipation of induced charges of the same polarity as said deposited charges, the dissipation of said induced charges being within the sphere of influence of said deposited charges to eifect retarding field neutralization.

30. The method of claim 29 wherein said insulator layer overlies a photoconductive insulator layer, said induced charges being dissipated by photoconductivity from the interface between said photoconductive insulator layer and said insulator layer, said photoconductive insulator layer being of suflicient thinness suflicient having an electron mobility therein to prevent the formation of a trapped bulk negative charge. 7

31. The method of substantially neutralizing the retarding fields created by deposited image-forming charges comprising causing the deposition of said image-forming charges in imagewise configuration on an insulator surface and simultaneously therewith counter-balancing the deposition of said deposited charges with the formation of opposite polarity charges sufficiently adjacent said imageforming charges so that the retarding fields established thereby are substantially neutralized.

32. The method of claim 31 further including visualizing the latent electrostatic image formed on said insulator layer.

33. The method of claim 32 further including fixing said visualized image to said insulator layer.

34. The method of claim 32 further including transferring said visualized image from said insulator layer to a copy sheet and fixing said image thereon.

35. An imaging apparatus comprising a cathode and an anode in parallel spaced relationship thereto, said cathode being connected to said anode through an external potential source, said cathode having a layer of photo-emissive material coated on the side of said cathode adjacent said anode, said anode having an insulator layer disposed on the surface thereof adjacent said cathode, said cathode and said anode being spaced further apart than the combined thicknesses of said photo-emissive layer and said insulator layer to thereby define a gap therebetween said anode having on the opposite surface thereof a layer of low work function material.

36. An ionography imaging method comprising providing the imaging apparatus of claim 35, disposing an object to be radiographed between a source of imaging radiation and the exposed surface of said cathode, causing a quantity of quenching gas to be flowed through at least the gap between said cathode and said anode, exposing said object to imaging radiation while suflicient potential is maintained across said anode and said cathode to cause negative charge avalanching in the gap therebetween whereby negative image-forming charges are deposited on said insulator layer in imagewise configuration in proportion to the amount of radiation absorbed by said object, and simultaneously therewith causing said imaging radiation passing through to the low work function material layer disposed on the opposite surface of said anode from said insulator layer to eject electrons from said low work function material layer whereby the holes remaining after said electron ejection substantially neutralize the retarding fields created by negative charges deposited on said insulator layer.

37. An ionography imaging method comprising providing the imaging apparatus of claim 35, disposing an object to be radiographed between a source of imaging radiation and the low work function material coated surface of said anode, causing a quantity of quenching gas to be flowed through at least the gap between said cathode and said anode, exposing said object to imaging radiation while sufficient potential is maintained across said anode and said cathode to cause negative charge avalanching in the gap therebetween whereby negative image-forming charges are deposited on said insulator layer in imagewise configuration in proportion to the amount of radiation absorbed by said object, and simultaneously therewith causing said imaging radiation absorbed by said low work function material layer disposed on the surface of said anode to eject electrons therefrom whereby the holes remaining after said electron ejection substantially neutralize the retarding fields created by negative charges deposited on said insulator layer.

Reiss, Zeit Fur Angew. Physik, vol. 19, pp. 1 to 4, February 1965.

US. Cl. X.R. 250-495; 355-3, 17